conf: Add target type and model for pl011
    
    We can finally introduce a specific target model for the pl011 device
    used by mach-virt guests, which means isa-serial will no longer show
    up to confuse users.
    
    We make sure migration works in both directions by interpreting the
    isa-serial target type, or the lack of target type, appropriately
    when parsing the guest XML, and skipping the newly-introduced type
    when formatting if for migration. We also verify that pl011 is not
    used for non-mach-virt guests and add a bunch of test cases.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=151292
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>